The invention relates to a braking system having at least one brake actuated by the delivery of braking energy wherein the effect of hysteresis of the brake when changing from brake actuation to brake release is eliminated or greatly reduced.
A braking system is known from DE 35 02 825 A1 wherein the operator sets an operator signal by actuating a braking value demand mechanism (e.g., a brake pedal) and thereby informs the braking system of his braking value requirement. The relationship between braking value and operator signal can be represented as a desired-value characteristic line in a characteristic diagram wherein the braking value (B) and the operator signal (S) are the coordinates. FIG. 1 shows such a characteristic diagram schematically with the desired-value characteristic line (T) as a solid line. The desired-value characteristic line (T) is equal to zero only when the operator signal (S) is equal to a certain operator signal value SA because of unavoidable response resistances of the brake. In this braking system, the braking value (B) is a parameter which depends on the braking force produced by the brake. The braking value (B) is the braking force itself, when it deepends exclusively on the braking force. However, in most cases the braking value (B) is a parameter which is determined by parameters of the braked object in addition to the braking force. In the field of automotive technology, the braking force between wheel and road surface and the vehicle deceleration are frequently used as the braking value. Thus, the dimensions of the vehicle or of vehicle parts, or the vehicle weight, contribute to the braking value. In the known braking system of the above-mentioned DE 35 02 825 A1, the vehicle deceleration, the braking force between wheel and road surface, and the utilization of the frictional interaction between wheel and road surface are designated as the braking force. A control unit transmits a braking value signal to an energy dimensioning device upon receiving an operator signal, whereupon the energy dimensioning device supplies braking energy to the brake as a function of the magnitude of the braking value signal, but often also as a function of additional signals as well. The brake then delivers a braking force which corresponds to the delivered braking energy. In the known braking system, the control unit always dimensions-the braking value signal so as to be equal or proportional to the operator signal.
Due to hysteresis, the brake's characteristic lines for the delivered braking force (K) follow different courses as a function of the braking energy (Z) for brake actuation and for brake release. This is shown in the schematic braking force/braking energy diagram according to FIG. 3. When the brake is actuated, i.e., as the supply of braking energy (Z) rises or remains unchanged, the braking force (K) follows characteristic line (X), starting from a braking force value ZA which is required to overcome the brake response resistances. When the brake is released, i.e., as braking energy (Z) drops, the braking force (K) follows the course of characteristic line (Y). Due to brake response resistances, the braking force (K) becomes equal to zero only in the presence of a residual braking energy ZR. For purposes of analysis, it shall be assumed that the brake is actuated up to a point with the coordinates of braking energy Z1 and braking force K1. If the braking energy (Z) is now decreased in order to release the brake, this does not have any effect upon the braking force K1 until the braking energy (Z) overcomes the difference in characteristic lines at a value Z2 and reaches characteristic line (Y) for brake release. Only as the braking energy (Z) continues to drop below Z2 does the braking force (K) decrease according to characteristic line(Y) for brake release. The characteristic line difference mentioned here is the hysteresis associated with point Z1/K1, and the surface between the characteristic lines (X) and (Y) is the hysteresis field.
This property of the brake affects the braking system also in the sense that the braking value (B) produced by the braking system, as a function of the braking value signal (aS) follows different characteristic lines for brake actuation and for brake release. This is shown in FIG. 2 through the corresponding characteristic lines (V) and (W). In FIG. 2 the characteristic line difference or surface between the characteristic lines (V, W) represents the hysteresis or the hysteresis field of the braking system. Similarly to FIG. 3, the braking value signal values aSA and aSR represent starting and finishing points of the characteristic lines (V, W) resulting from brake response resistances and the residual energy required to overcome the former. For example, if the braking system is actuated up to a point with the coordinate braking value signal aS1 and braking value B1, and the braking value signal (aS) is then decreased, the characteristic line difference or hysteresis associated with point aS1/B1 must be overcome before the braking system responds to the decrease in braking value signal (aS) and lowers the braking value (B). Because of the previously-mentioned permanent maintenance of equality or proportionality between between opertor signal(S) and braking value signal (aS), the just described behavior of the known braking system in dependency on the braking value signal (aS) means an identical or similar behavior in dependency on the operator signal (S). In other words: For brake release, the operator signal (S) must first drop by a value dependent on the hysteresis before the braking system responds with a braking value drop. The "reactionless drop" of the operator signal (S) which is required to overcome the hysteresis and thereby to obtain a reaction of the known braking system is also accompanied by a delayed response when the brake is released.